The purpose of a gust response testing is to examine the effect of time dependent flow on an aircraft and, in this manner, to avoid the excitation of the structural modes by gusts and simultaneous pilot commands. These factors can result in degraded handling qualities and inadequate fatigue life. In addition, gust tunnels are employed to produce the time dependent flow to which turbomachinery components are exposed.
The majority of the techniques used to produce the time dependent flow required in gust tunnels are mechanical. One of the earlier gust tunnels consisted of two airfoils mounted in a biplane arrangement in each sidewall of the wind tunnel upstream of the test section. The airfoils were connected to a heavy flywheel and thereby driven in an oscillatory fashion in terms of angle of attack. This oscillation produced trailing vortices of alternating signs from each of the airfoils. The trailing vortices travelled downstream near the sidewalls of the tunnel and induced either an upwash or a downwash depending on the instantaneous sign. The frequency limit of the system is 20 Hz. One limitation to the tunnel is that the flow induced by the trailing vortices leads to a nonuniform transverse velocity profile across the tunnel span.
Another method of testing a model aircraft has the model passing through a single transverse gust with the model mounted on a sled traveling on a high speed track through an open jet wind tunnel directed normal to the line of travel of the model. The tunnel efflux exposes the model to a single large uniform transverse gust, which is well defined but does not allow the excitation of structural modes since it is not repeated. The model tests, as well as full scale aircraft tests, were compared to theoretical predictions.
Another attack on the problem consists of flexible test section walls on the wind tunnel. Nearly sinusoidal traveling waves are created on the top and bottom flexible walls by a system of cams and springs. Two different types of gusts, with either transverse or streamwise variations, can be produced. If the traveling waves on the top and bottom walls are in phase, the entire tunnel flow moves up and down with the walls and produces a transverse gust. If, however, the traveling waves are 180.degree. out of phase, then the effect produced is a streamwise gust.
Another approach to gust tunnel testing involves a mechanical oscillation of the entire inlet to a small wind tunnel. A cascade is mounted in the test section and guide vanes are mounted in the inlet, which is driven by an eccentric cam. The resulting oscillatory flow produced in the test section is nominally of constant magnitude and varying direction. Therefore, this concept has great potential for the study of turbomachinery components. The primary limitations of the process appears to be the inertial problems posed by either high frequencies (requiring very high accelerations) or larger dimensions (in which case large masses must be accelerated). Current operating frequencies are on the order of 20 Hz.
A method capable of producing high frequency oscillations in a wind tunnel is described in the patent to Jacobs et al., U.S. Pat. No. 3,669,386. The basic mechanism is a fluidically controlled oscillating jet which flaps from side to side. On either side of the power jet are two control jets which are alternately cycled on and off by a remotely located servo-motor driven signal and thereby cause the power jet to attach to one side or the other. The resulting flapping jets are then installed in a wind tunnel and oscillate in phase, thereby diverting the tunnel flow in an oscillatory manner. The basic advantages of this system are its simplicity and the potential to achieve rather high frequencies.
The capability to produce high frequency gust tunnel oscillations is also desirable from the point of view of random gust work because the energy spectrum of atmospheric turbulence extends over a wide frequency range. In experimental investigations, the usual procedure is to study the response of a complex aircraft model to the gust of a preassigned frequency. From the statistical standpoint, the procedure is to investigate the model's response to random gusts of quite different frequencies. When the aircraft dynamics are assumed linear, the effect of random gusts can be synthesized by adding up the individual responses over a wide frequency range. This therefore amounts to experimentally determining the transfer function of the allegedly linear aircraft motion.